The book published by Siemens AG, "Elektronikschaltungen" ("Electronic Circuits"), article by W. Hirschmann, page 148, shows a suitable circuit for operating fluorescent lamps from a-c power networks by rectifying the received a-c network power and converting it to high frequency, to obtain more efficient light output upon operation of the fluorescent lamp. The circuit as known, as well as the circuit of the present invention, is not restricted to operating only a single fluorescent lamp; the circuits can operate also a plurality of fluorescent lamps, connected in parallel or in series. If operated in parallel each one of the fluorescent lamps will have an individual resonant circuit associated therewith.
The circuits to generate the high frequency use switching transistors. The switching transistors are so connected that the emitters have emitter resistors serially connected thereto, the emitter-resistors having a value of from about 0.5 to 5 ohms. These emitter-resistors, which are connected to each one of the switching transistors, are used to compensate for differences in individual characteristics of transistors, to stabilize the thermal operation of the transistors and, in some instances, to permit adjustment of the operating power being supplied to the fluorescent lamp with which the circuit is used. The resistors, through which the emitter current flows, result in localized current inverse feedback. This inverse feedback reduces the current amplification of the transistors and, for a given base drive of the transistors, limits the maximum current which can flow therethrough, that is, the maximum generated current of the oscillator and thus also the maximum voltage which can be generated by resonant voltage amplification.
Some types of lamps require substantial ignition voltages. It is frequently difficult and sometimes impossible to obtain a sufficiently high generator voltage to ensure reliable firing or ignition of the lamps without additional substantial circuitry in the starting and operating circuit. These difficulties are particularly noticeable when the lamps are to be operated from power networks of nominally 100 to 120 V. The voltage enhancement over the nominal voltage to ensure reliable ignition of the lamps, and hence starting of the lamps, must be greater than when the lamps are operated from higher voltage networks.
U.S. Pat. No. 4,553,070 describes a circuit in which, in series to the emitters, diodes are connected which have the purpose to increase the time during which both transistors are OFF by providing a threshold value which has to be exceeded by the drive voltage. This increases the gap between conduction of the respective transistors, and prevents the flow of stray or cross currents.